In several applications there is a demand for accurately manufactured solenoidal coils, which have high mechanical and electrical integrity.
This demand exists, for example, in transverse flux motors (TFMs), which are permanent magnet machines under development for ship propulsion.
In contrast to the geometrically complex electrical stator windings of conventional motors, a TFM has a simple solenoidal (i.e. substantially circular) coil. Although this shape is simple geometrically, its manufacture and operation at high voltages require new manufacturing and installation techniques.
The space available for the coil in the overall TFM is generally limited, and therefore the coil dimensions must be tightly controlled. Attention must also be paid to the means of connecting the coil to the power supply, in order to effect a reliable connection, which requires minimum space to take the electrical connection through the machine structure to the external motor terminals.
For all solenoid-type coils, electrical integrity is important. A TFM typically operates at around 3 kV and is normally driven by a pulse width modulated inverter that switches at high speeds (several kHz). The fundamental frequency of the motor at full speed can be around 200 Hz. Together these factors result in an arduous electrical duty and require a coil design which should reduce voltage stress within the coil windings and eliminate corona discharge, factors which could result in premature coil failure in use.
Operation of electrical coils at high voltage and high switching rates such as those encountered when using inverter based drives places significant demands on the coils. Previous machines have suffered insulation failure as a result of corona discharge and high dV/dt between adjacent windings and between the first and second turns in each winding.
Good manufacturing techniques can mitigate or even avoid these effects. Such techniques include pressing and curing the conductors or wires of the coil, which are usually pre-wrapped with epoxy-loaded tape; avoiding air voids in the insulation system and the application of a semiconductive ground plane wrap on the outside of the coil. A function of the ground wrap is to provide a defined high resistance leakage path for leakage currents which pass through the conductor insulation, avoiding charging of the outer coil surface to a high voltage which would otherwise lead to corona or arcing discharge to ground and damage to the insulation.
These techniques have been employed previously on conventional stator coils, which have long straight sections of conductor or wire which are ultimately located in stator slots. Means of applying the necessary compression to such sections of coil during the manufacturing process are well established for these conventional coils.